Update the LDAP_BACKGROUND_SYNC_INTERVAL setting documentation

docker-compose.yml

@@ -441,8 +441,9 @@ services:
       #- LDAP_BACKGROUND_SYNC=false
       #
       # At which interval does the background task sync in milliseconds.
-      # Leave this unset, so it uses default, and does not crash.
-      # https://github.com/wekan/wekan/issues/2354#issuecomment-515305722
+      # The format must be as specified in:
+      # http://bunkat.github.io/later/parsers.html#text
+      # The default is "every 1 hour"
       - LDAP_BACKGROUND_SYNC_INTERVAL=''
       #
       #- LDAP_BACKGROUND_SYNC_KEEP_EXISTANT_USERS_UPDATED=false